In the description of the background of the present invention that follows reference is made to certain structures and methods, however, such references should not necessarily be construed as an admission that these structures and methods qualify as prior art under the applicable statutory provisions. Applicants reserve the right to demonstrate that any of the referenced subject matter does not constitute prior art with regard to the present invention.
Hard metals based on tungsten carbide include composites consisting of small (μm-scale) grains of at least one hard phase in a binder phase. In these materials, the hard phase tungsten carbide (WC) is always present. In addition, other metal carbides with a general composition (Ti,Nb,Ta,W)C may also be included, as well as metal carbonitrides, e.g., Ti(C,N). The binder phase usually consists of cobalt (Co). Other binder phase compositions may also be used, e.g., combinations of Co, Ni, and Fe, or Ni and Fe.
Industrial production of tungsten carbide based hard metals often include blending of given proportions of raw materials and additives with a milling liquid. This liquid is often an alcohol, e.g., ethanol, water, or a mixture thereof. The blend is then milled into a homogeneous slurry. The wet milling operation is made with the purpose of deagglomerating and intimately mixing the raw materials. Individual raw material grains are also disintegrated to some extent. The slurry is then dried and granulated, e.g., by means of a spray drier. The granulate may then be used in uniaxial pressing of green bodies, generally with PEG (polyethylene glycol) added as pressing agent. Alternatively, stearic acid may be used as an additive when the material is to be used for extrusion or injection molding.
Injection molding is common in the plastics industry, where material containing thermoplastics or thermosetting polymers are heated and forced into a mold with the desired shape. The method is often referred to as Powder Injection Molding (PIM) when used in powder technology. The method is expensive compared to uniaxial pressing and is hence preferably used for parts with complex geometry.
In powder injection molding of tungsten carbide based hard metal parts, four consecutive steps are applied:
1. Mixing of the hard metal raw materials with a polymer system. The polymer system acts as a binder and constitutes 25–55 volume % of the resulting compound. The exact concentration is dependent on the desired process properties during molding. The mixing is made with all organic constituents in molten state in an extruder. The resulting compound is obtained as pellets of approximate size 2×4 mm.
2. Injection molding using the compounded feedstock. The material is heated to 100–240° C. and then forced into a cavity with the desired shape. The part is cooled and then removed from the cavity.
3. Removing the binder from the obtained part. The removal can be obtained by wet extraction of the parts or by heating in a furnace with suitable gases present, or a combination thereof.
4. Sintering of the parts. Common sintering procedures for hard metals are applied.
The above-mentioned processes require the use of alcohol, energy, equipment time, and manpower. It would be desirable to reduce these needs for economical and ecological reasons.
The article “Dispersing WC-Co powders in aqueous media with polyethylenimine” (E. Laarz and L. Bergstrom, International Journal of Refractory Metals & Hard Materials, 18, 2000, pp. 281–286) gives an account of PEI (polyethylenimine, a cationic polyelectrolyte) in slurries of tungsten carbide and cobalt in water. PEI acts as a dispersant at concentrations above 0.3% with respect to dry powder weight (wt %).
EP-A-1153652 relates to the preparation of dispersed suspensions of WC and Co in water or water-ethanol mixtures using PEI. In water based slurries with 3.5 wt % PEG present, a dispersing effect of PEI was reported at concentrations above 0.3 wt %. For mixtures of water and ethanol, the lowest concentration of PEI stated to have dispersing effect is 0.3 wt %. The slurry is made from a mixture of 90 wt % ethanol and 10 wt % water with WC, TiC, TaC, TiN, and Co powders. A concentration range of a polyethylenimine-based polyelectrolyte of 0.1–10 wt % is claimed.
Swedish patent application 0104309-0 relates to the addition of low concentrations (0.01–<0.1 wt %) of PEI to slurries containing ethanol, water, PEG, and powdered raw materials for the production of tungsten carbide based hard metals. A radical decrease in slurry viscosity is thus obtained, which can be used to decrease the volume of milling liquid, the milling time, the rinsing liquid volume, and energy use on slurry drying. A concentration range of 0.01–<0.1 wt % of a polyethylenimine-based polyelectrolyte is claimed.